A publication of
CHEMICAL ENGINEERING TRANSACTIONS
The Italian Association
of Chemical Engineering
www.aidic.it/cet
VOL. 35, 2013
Guest Editors: Petar Varbanov, Jiří Klemeš, Panos Seferlis, Athanasios I. Papadopoulos, Spyros Voutetakis
Copyright © 2013, AIDIC Servizi S.r.l.,
ISBN 978-88-95608-26-6; ISSN 1974-9791
Process intensification alternatives in the DME production
Anton A. Kiss*a, David J-P. C. Suszwalakb, Radu M. Ignatc
AkzoNobel – Research, Development & Innovation, Process Technology ECG, Zutphenseweg 10, 7418 AJ, Deventer,
The Netherlands, Tel: +31 26 366 9420, E-mail: tony.kiss@akzonobel.com
b
Ecole Nationale Supérieure de Chimie de Mulhouse (ENSCMu), Mulhouse, France.
c
University “Politehnica” of Bucharest, Department of Chemical Engineering, Polizu 1-7, 011061 Bucharest, Romania.
a
The increasing demand for dimethyl ether (DME) requires novel technological solutions able to overcome
the drawback of energy intensive distillation steps, and to reduce the overall costs of the current process.
This study provides a brief overview of process intensification alternatives for the DME production based
on dividing-wall column (DWC) technology and reactive distillation (RD). Rigorous simulations were
performed in AspenTech Aspen Plus, where all alternatives based on DWC, RD and R-DWC, were
optimized using sequential quadratic programming (SQP). The newly proposed processes allow significant
energy savings, while using less equipment units and simplifying the overall process operation.
1. Introduction
Dimethyl ether (DME) is of high industrial interest due to its use as clean fuel for diesel engines or in
combustion cells, as a precursor to other organic compounds, and as a green aerosol propellant that can
effectively replace chloro-fluoro-carbons (CFC). Presently, dimethyl ether is produced by the conversion of
feedstock such as natural gas, coal, oil residues and bio-mass into syngas (CO and H2), followed by a twostep process: methanol synthesis and then methanol dehydration. Figure 1 (left) illustrates the simplified
conventional flowsheet for DME production by methanol dehydration: 2 CH3OH  CH3-O-CH3 + H2O
Methanol is produced first from syngas over a copper-based catalyst, and then it is dehydrated over a γalumina catalyst or zeolites in order to produce DME. Different types of solid acid catalysts can be used,
such as γ-alumina (γ-Al2O3), HZSM-5, silica-alumina, phosphorous- and fluorinated-alumina. Among
them, γ-alumina is the preferred one due to its thermal stability, mechanical resistance, high surface area
and catalytic properties. The methanol dehydration step takes place at temperatures of 250-400 °C and
pressures up to 20 bars (Muller and Hubsch, 2005). The industrial process involves a fixed-bed catalytic
reactor (gas-phase), followed by a direct sequence of two distillation columns delivering very high-purity
DME (99.99 %wt), recovering methanol and discharging the water by-product. Note that the conversion of
methanol lies within the 70-80% range, depending on the catalyst and the operating conditions. Due to the
incomplete conversion, the outlet of the reactor consists of a ternary mixture: DME, water and methanol.
This mixture is cooled and subsequently distilled in the first tower to yield pure DME, while the un-reacted
methanol is separated from water in a second distillation column, and then recycled back to the reactor.
DME
RX
DC1
DME
Methanol
(recycle)
Feed
DC2
Methanol
Methanol
Steam
Fixed-bed reactor
(gas phase)
Water
Figure 1. Conventional DME process (left) and separation alternative based on DWC (right).
Water
DME
Methanol
(recycle)
Methanol
DME
R-DWC
Methanol
Methanol
(recycle)
DC
RDC
Water
Water
Figure 2. Alternative processes based on reactive distillation (left) or reactive DWC (right).
This paper gives a brief overview of several process intensification technologies that could be applied to
improve the conventional process, by combining several functions or tasks (e.g. reaction, distillations) into
one piece of equipment. A very innovative solution to overcome the drawback of energy intensive
distillation is using dividing-wall column (DWC) technology that can save up to 30% in CapEx and up to
40% in OpEx (Dejanovic et al., 2010; Yildirim et al., 2011). Figure 1 (right) illustrate such a process
alternative where the classic direct distillation sequence is carried out in a DWC unit (Kiss and Ignat,
2013). Moreover, Figure 2 shows other process alternatives that are based on reactive distillation (An et
al., 2004; Lei et al., 2011) or a reactive dividing-wall column combining 2-3 functions or tasks into one
integrated unit: e.g. chemical reaction, DME separation, methanol recovery (Kiss and Suszwalak, 2012).
Remarkable, DWC technology is very versatile and it can be used also in extractive distillation, azeotropic
separations, and reactive distillation (Mueller and Kenig, 2007; Hernandez et al., 2009; Kiss et al., 2009,
2012; Yildirim et al, 2011). The design, control and applications of DWC are nowadays quite well
established (Dejanovic et al., 2010; Yildirim et al, 2011; Kiss and Bildea, 2011). For a fair comparison, all
the new DME process alternatives are optimized in terms of minimal energy requirements, using the state
of the art sequential quadratic programming (SQP) method implemented in Aspen Plus (Boggs and Tolle,
1995). The results presented hereafter clearly demonstrate that significant energy savings are possible,
while less equipment units are needed as compared to the conventional process configuration.
2. Problem statement
The use of DME as clean fuel and green aerosol propellant received much scientific interest associated
with a growing industrial demand for higher DME production rates at lower production costs. Traditionally,
high purity DME is synthesized by dehydration of methanol produced from syngas in as conventional gasphase process that involves a catalytic fixed-bed reactor followed by a direct sequence of two distillation
columns. The main problem of this process is the high investments costs for several units (e.g. reactor,
columns, heat exchangers) that require a large overall plant footprint, as well as the associated energy
requirements. Consequently, better process alternatives are needed in order to reduce the overall costs.
To solve these problems, we present here novel DME process intensification alternatives – based on DWC
or / and RD – that use a reduced footprint and allow significant savings in investment and operating costs.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Molefrac DME
Figure 3. Ternary diagram (left) and residue curve map (right) for the mixture DME-methanol-water
0.8
0.4
0. 2
0. 3
0. 4
0. 5
0. 6
0. 1
0. 9
0. 2
0. 8
0. 3
0. 7
0. 4
0.10.20.30.40.50.60.70.80.9
DME
(4 4.40 C)
0.2
0. 1
ER
le f
rac
WA
T
Mo
0.8
0.6
L
NO
HA
0. 5
0.4
ET
0. 6
0.2
M
rac
le f
0. 7
DC1
0. 8
WATER
(1 79.98 C)
DC2
0. 9
RX
METHANOLT
0.6
Mo
Residue curve for DME/METHANOL/WATER
METHANOLTernary
(136.8 1Map
C) (Mole Basis)
WATER
(1 79.98 C)
200
1
DME
180
160
Main column
140
Mass fraction / [-]
Temperature / [°C]
Water
Methanol
0.8
120
Prefractionator
100
80
60
0.6
0.4
0.2
40
20
0
0
5
10
15
20
25
Stage / [-]
30
35
40
45
0
5
10
15
20
25
Stage / [-]
30
35
40
45
Figure 4. Temperature (left) and composition profiles (right) of the DWC used for single-step separation
3. Results and discussion
Rigorous simulations were carried out in Aspen Plus for the base case scenarios as well as the DWC and
RD alternatives proposed. UNIQUAC-Redlich-Kwong was selected as an adequate property method, and
the binary interaction parameters were validated against reported experimental data (Teodorescu and
Rasmussen, 2001; Ihmels and Lemmon, 2007). The ternary map and the residue curves map (RCM) of
the DME-methanol-water mixture (Figure 3) – indicate that no azeotropes are present, but a small liquid
phase split envelope is observed, hence all distillation columns are modeled using VLLE data. The ternary
map also gives an indication of the main tasks involved in the DME process: chemical reaction (RX)
followed by DME purification (DC1) and methanol recovery combined with water removal (DC2).
It is worth noting that systems consisting of reactor-separator-recycle are prone to exhibit multiple steady
state and nonlinear behavior (Kiss et al., 2005, 2007). Therefore, the integrated design and control of such
systems is of utmost importance – although this topic is outside the scope of this paper. Nevertheless,
these undesired phenomena can be avoided if the reactor inlet is set on flow control and the fresh feed is
on self-regulation control (Kiss et al., 2007).
3.1 Single-step separation
The DWC unit replacing the direct sequence of an industrial process (100 ktpy) was designed using
heuristic rules and then employing the sequential quadratic programming (SQP) optimization method and
the sensitivity analysis tool available in AspenTech Aspen Plus (Boggs and Tolle, 1995).
Table 1. Design and operating parameters of a DWC for single-step DME separation (100 ktpy)
Design parameters
Flowrate of feed stream
Feed composition (molar fractions)
DME : Methanol : Water
Temperature of feed stream
Pressure of feed stream
Operating pressure
Column diameter
Number of stages pre-fractionator side
Total number of stages DWC
Feed stage pre-fractionator
Side stream withdrawal stage
Wall position (from / to stage)
Distillate to feed ratio
Reflux ratio
Liquid split ratio (rL)
Vapor split ratio (rV)
DME product purity
Methanol recycle purity
Water product purity
Reboiler duty
Condenser duty
Value
22880
Unit
kg / hr
0.38 : 0.24 : 0.38
120
10
10
1.7
17
42
12
20
16-32
0.546
3.87
0.27
0.42
99.99 / 99.99
99.10 / 98.40
99.99 / 99.99
4028
–6322
–
°C
bar
bar
m
–
–
–
–
–
kg / kg
kg / kg
kg / kg
kg / kg
%wt / %mol
%wt / %mol
%wt / %mol
kW
kW
Table 2. Head-to-head comparison of conventional separation sequence vs DWC alternative
Conventional
separation
$1,760,752
$1,388,550
$1,564,625
448.0
569.7
Key performance indicators
Total investment cost (TIC)
Total operating costs (TOC)
Total annual costs (TAC)
Specific energy requirements (kW·h/ton DME)
CO2 emissions (kg CO2/h·ton DME)
DWC
alternative
$1,412,490
$997,735
$1,138,984
322.2
409.8
Table 1 provides the design and operating parameters of the optimal DWC unit, while Figure 4 illustrates
the temperature and composition profiles along the DWC (Kiss and Ignat, 2013). DME and water are the
top and bottom end high purity products (>99.99 %wt), while methanol accumulates towards the middle of
the column, being withdrawn as a side stream (>99 %wt) and then recycled. The temperature difference
between the two sides of the wall is rather low – such conditions being feasible for practical
implementation, with negligible effect on the column performance (Yildirim et al., 2011).
Table 2 provides a head-to-head comparison of the key performance economic indicators. Remarkable,
the DWC alternative requires less equipment and 20% lower capital costs, while being the most energy
efficient allowing significant energy savings of over 28%, as compared to the conventional distillation
sequence considered here. Note that the specific energy required per ton of DME product is much lower
than the earlier reported value of 513.75 kWh/ton (1849.53 kJ/kg) of DME, for the distillation step alone
(Lei et al., 2011). Revamping the direct separation sequence to a DWC is also possible for existing
industrial plants. When this option is considered, about 28% lower TAC is possible while requiring much
less investment costs as compared to the case of building a new DWC (Kiss and Ignat, 2013).
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
RDC
Molar fraction / [-]
Molar fraction / [-]
3.2 Reactive distillation process
For a pilot plant scale DME plant (9 kmol/hr, or 3300 ton/year) we consider an alternative DME process
based on reactive distillation. The RD column is divided into three sections with a central reactive zone
from which the products are continuously removed thus overcoming the equilibrium limitations. High-purity
DME is collected at the top of RDC, while a mixture water-methanol is obtained as bottom stream that is
afterward fed to a distillation column (Figure 2 left). Pure water is delivered as bottom stream of the DC,
while the top distillate consists of mainly methanol and tiny amounts of DME. The top methanol stream is
then conveniently recycled back to the RDC unit. RDC consists of 32 stages with the reactive zone located
from stage 8 to 31. The methanol feed stream is fed close to the top of the reactive zone, on stage 12.
Within the reactive zone of the RDC, a total load of 15 kg of solid catalyst per stage was used. The
following distillation column (DC) for water separation and methanol recovery has a total number of 23
stages with the feed located on stage 17. The stages are numbered from top to bottom, thus stage 1 being
the condenser and stage 32 the reboiler.
Despite a rather high investments cost required for 2 columns, 2 reboilers and 2 condensers, this RD
process has the key advantage of being flexible as the RDC and DC can both be operated at different
pressures – this not being possible in a DWC configuration. Note that here the RDC is operated at 10 bar
while DC is operated at 1 bar, just as in the classic process.
Figure 5 plots the temperature and liquid composition profiles of along the RDC and DC (Kiss and
Suszwalak, 2012). Since both distillation columns show similar temperature ranges, the use of a DWC
seems to be an attractive alternative. High purity (>99.99 %wt) DME and water products are obtained
whereas the purity of the recovered methanol stream that is recycled is also high (99.9 %wt). Remarkable,
the methanol conversion is slightly above 50% as the reaction takes place in liquid phase, and the overall
specific energy requirements account for 1.37 kWh/kg DME.
Methanol
DME
Water
0
5
10
15
Stage / [-]
20
25
30
35
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
DC
Methanol
DME
Water
0
5
10
15
Stage / [-]
Figure 5. Composition profiles along the RDC and DC units (RD in a two-column sequence)
20
25
180
160
Molar fraction / [-]
Temperature / [°C]
Side product section
RD side of DWC
200
140
120
100
80
Temperature side section
60
Temperature R-DWC
40
0
5
10
15
20
25
30
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
DME
0
35
Methanol
5
10
15
20
Water
25
30
35
Stage / [-]
Stage / [-]
Figure 6. Temperature and composition profiles along the reactive DWC (dashed line used for the side
product section, while continuous line used for the main DWC section)
3.3 Reactive DWC process
All units of the conventional DME process (reactor and two distillation columns) can be integrated all
together in a reactive DWC consisting of only one column shell, one reboiler and one condenser (Figure 2,
right). The main condition in integrating two distillation columns is that similar operating conditions should
be applied. The Aspen Plus model for the RDC + DC sequence was used as the starting point for the RDWC simulation, providing initial estimates for the number of trays, feed tray locations, liquid and vapor
split and size of the reactive zone. Figure 6 plots the temperature and composition profiles in the R-DWC
(Kiss and Suszwalak, 2012). The R-DWC unit has 35 stages, with the reactive zone located from stage 8
to 31 on the feed side, and a common stripping section (stage 32 to 35) as well as a common rectifying
zone (stage 1 to 7). The methanol stream is fed on stage 8, at the top of the reactive zone – the feed side
of the DWC acting as the RD zone where the solid acid catalyst is present. High purity (>99.99 %wt) DME
is delivered as top distillate, while similar high-purity water is obtained as bottom product.
Remarkable, the temperature difference between the two sides of the wall is very low – less than 15 °C –
such conditions being achievable in the practical application. Note that the feed side of the DWC acts as
the reactive distillation zone where the solid acid catalyst is present. The feed stream is located on stage
8, at the top of the reactive zone of the 35 stages R-DWC. High purity (>99.99 %wt) DME is delivered as
top distillate, while similar high-purity water is obtained as bottom product. The unreacted methanol is
collected as side product, and then recycled back to the process – mixed with the fresh feed stream of
methanol. The profiles are very similar to the reactive distillation system previously described, with sharp
modifications in the temperature and the composition profiles around the feed location – between stages 5
and 10. On the side product part, methanol concentration remains almost constant on a large range of
stages (10-20) thus indicating that the side stream location has only a minor effect on the products purities.
While the profiles of the RD and R-DWC processes are similar, the key difference is the higher stripping
section required in case of R-DWC for methanol recovery. The methanol conversion is about 50% but with
much lower specific energy requirements, of only 0.56 kWh/kg DME.
Remarkable, the reactive DWC process is the most energy efficient allowing energy savings of 11.6% and
58.6% as compared to conventional and RD processes, respectively – see Figure 7 (Kiss and Suszwalak,
2012). Moreover, the R-DWC process is also the one using the least equipment units. Although the RD
process has higher energy requirements compared to the conventional process, the former should be
preferred for its low footprint and milder operating conditions – e.g. lower temperature levels.
140,000
1.37
1.4000
1.2000
100,000
1.0000
0.8000
TIC
TOC
TAC
120,000
0.64
0.57
0.6000
Cost / [US $]
Specific energy use / [kW.h/kg DME]
1.6000
80,000
60,000
40,000
0.4000
20,000
0.2000
0
0.0000
Conventional
RD+DC
R-DWC
Conventional
RDC+DC
R-DWC
Figure 7. Comparison of DME processes in terms of key performance indicators (3300 ton/year plant)
4. Conclusions
Reactive distillation and dividing-wall column technology can be effectively used for improving existing and
new DME processes. For example, the conventional DME purification and methanol recovery distillation
sequence can be successfully converted into a single-step separation based on DWC. As compared to the
conventional direct sequence of two distillation columns, the novel proposed DWC alternative reduces the
energy requirements by 28% and the equipment costs by 20%.
Moreover, reactive distillation is a feasible process intensification alternative to produce DME by methanol
dehydration, using solid acid catalysts. The innovative reactive DWC process has excellent performance
with significant energy savings of 12-58%. Consequently, the R-DWC process can be considered as a
serious candidate for the DME production in new as well as revamped industrial plants.
All the new separation schemes also require less equipment units and reduced plant footprint – thus
sparing existing equipment (column shell, heat exchangers), usable elsewhere in the chemical plant.
Acknowledgments
The support given by AkzoNobel to David Suszwalak (ENSCMu, France) during his MSc internship and to
Radu Ignat (Politehnica University of Bucharest) during his PhD internship, as well as the financial support
from the Sector Operational Program Human Resources Development 2007-2013 of the Romanian
Ministry of Labor, Family and Social Protection through the Financial Agreement POSDRU/88/1.5/S/61178
are gratefully acknowledged.
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